Device for floating zone-melting of semiconductor rods



W. KELLER Oct. 22, 1963 DEVICE FOR FLOATING ZONE-MELTING OF SEMICONDUCTOR RODS Filed Aug. 12. 1960 PRIOR ART em iamc'zz Fig. 1

DOC)- Fig. 2

United States Patent BEVBCE FQR FLOATENG ZlltNE-h lELTlNG til SEMICONDUCTGR RGDS Wolfgang Keller, lretsteld, Upper Franconia, Germany, assignor to Siemens-Schuclrertwerlre Alrticngesellsehart, licrlimsiemensstadt, Germany, a corporation or Gen many Filed Aug. 12, 19%, Eden. No. 49,323

Claims priority, application Germany Aug. 14, 1959 9 Claims. (til. 2l9-3ltl.49)

My invention relates to a device for floating zone-melting of semiconductor rods, particularly as required for producing semiconductor wafers for rectifiers, transistors, photodiodes and other electronic components. The manufacture of such components involves the production of large quantities of hyper pure semiconductor material, for example silicon, germanium, or inter-metallic compounds of certain elements from the third and fifth groups of the periodic system such as indium antimonide, or indium arsenide. The latter are known in the art as A B semiconductors. One of the methods developed for producing such hyper pure semiconductor materials is the floating zone-melting.

According to this method, a rod of semiconductor material is inserted at its two ends into respective holders, being preferably mounted in vertical position. A ringshaped heating device, surrounding the rod and comprising in most cases an induction coil, is passed axially over the entire length of the rod while the rod portion located within the ring-shaped heater is melted. As the molten zone travels along the rod axis, new semiconductor material is continuously liquefied at the front of the zone, whereas a corresponding amount of semiconductor material solidifies at the opposite side. Due to the crystalline structure of the material, this melting and freezing is accompanied by a purification of the material from contaminations which renders the crucible free zone-melting suitable for purifying purposes.

Another application of the same method is the growing of monocrystals. For this purpose, a monocrystal seed is fused to one end of a polycrystalline semiconductor rod, and a melting zone, starting from the fused junction of the seed is passed through the entire length of the rod, if desired, in several repeated passes. The cruciblefree zone-melting method is also applicable for zonelevelling of impurities in which the molten zone is repeatedly passed through the entire rod length in both directions of travel, whereby the concentration of certain impurities contained in or added to the semiconductor mater-ial is uniformly distributed over the entire rod length. Such impurities which are to be equalized throughout the length of the rod may consist for example of boron in a semiconductor rod of silicon, boron having a distribution coefficient of nearly unity value relative to silicon. When the zone-melting is performed in an evacuated vessel, the above-described phenomena are accompanied by an additional purifying eflect, due to evaporation of foreign substances from the semiconductor rod.

The above-described crucible-free (floating) zonemelting method is predicated upon the phenomenon that the molten zone is held in position by surface tension. When the quantities'of molten material contained in the molten zone become too large, the surface will tear and the molten zone will drip off. For this reason, the method is limited to a given diameter of the rod being processed, beyond which the danger of the molten zone running oil becomes too great and the attention required of the supervisory personnel becomes excessively demand- It is an object of my invention, which relates to a floating zone-melting device for vertically mounted semicon- 3,198,159 Patented Got. 22, 1963 ductor rods having for example a ring-shaped induction heater surrounding the rod, to improve said device to reliably permit the zone-melting of rods having a larger diameter than heretofore permissible, particularly rods the diameter of which is greater than 15 millimeters.

According to a preferred embodiment of my invention, I provide a closed ring of electrically conducting material which, like the heater coil proper, surrounds the semiconductor rod, and is mounted beneath the heater coil at a fixed axial spacing relative thereto. The conducting ring is preferably made of silver, which possesses a good electrical conductance and can also be produced with sufiiciently high purity, so that no contamination of the semiconductor material by the ring is encountered.

The invention will be further described with reference to the accompanying drawing in which:

FIG. 1 illustrates the formation of a melting zone in a vertically mounted semiconductor rod, as occurring in thed zone-melting device of the type heretofore available; an

FIG. 2 is a partly sectional view of a device according to the invention also indicating the formation of the melting zone.

FIG. 3 shows another embodiment of the invention.

in the two first illustrations a semiconductor rod 1, for example of silicon, is surrounded by an induction heater coil 2. The rod is mounted at both ends in respective holders and the induction coil 2, during zone-melting operation, travels downwardly along the rod. The induction coil 2 is designed as a flat spiral in order to produce a narrow melting zone. The coil may consist for example of silveoplated tubes of copper traversed by cooling water during operation of the device, the flow of water being indicated by arrows. Said fiat coil and its supports and connections are more fully described in the co-assigned application of Keller and Emeis, Serial No. 23,535, filed April 20, 1960. These details are incorporated herein by reference. in the device according to the invention, as exemplified by the embodiment shown in FIG. 2, a short-circuit ring 3, consisting of an axially short hollow cylinder is mounted beneath the heater coil 2. The ring 3, consisting preferably of silver, is joined with a cooling coil 6 which is connected into the path of circulation of cooling water passing through the heater coil 2. However, the electrically good conducting ring 3 is not alone in electrical series with the heater coil 2, but is shunted by a conducting bridge member 4, which takes care that at least most of the electric heating current passing through the heater coil 2 is shunted around the ring 3.

As will be seen by comparing FIGS. 1 and Z, the melting zone 2 of the device of FIG. 1 tends to drip off, whereas this danger is eliminated in the device of FIG. 2 because the considerable axial constriction of the molten zone 5 reduces the size of the bulge at the lower end of the molten zone.

The just-mentioned reduction in axial width of the melting zone, is due to the fact that the heater coil induces an in the closed ring 3 which leads to a short-circuit current that has the effect of building up an opposite magnetic field within the ring which compensates for the field of the heater coil 2 in the semiconductor portion located within the ring 3. Consequently, no heating action is produced at such location shortly below the heater coil 2. As a result, only the rod portion located above the ring 3 can be melted.

The thus obtained axial reduction in Width of the melting zone greatly reduces the danger of the molten zone dripping off, and facilitates the desirable processing of rods having a relatively large diameter, in comparison with the rod diameters heretofore reliably permissible.

The just-mentioned danger is particularly pronounced when the heater coil travels downwardly. The known pear-shaped configuration of the melt then becomes particularly noticeable and the bulge at the lower end of the molten zone can tear olf. This is because, during downward travel, the effect of gravityis additive with respect to the magnetic field of the heater coil at the molten zone, and these additive eiiects are particularly great at the bulge of the zone. in the floating-zone melting devices heretofore available, the just-mentioned phenomena, as a rule, permit utilization of only the upward travel of the heater coil for zone-melting, when processing rods with a diameter above 15 mm. During subsequent downward travel, the heater coil is operated with reduced heating power, but sufiicient so that a glowing zone is kept alive, so that the zone-melting can again be istarted atter the heater coil has reached the lower end of a rod and its travel direction is then reversed. In contrast, the device according to the invention has the important advantage that it permits utilizing the upward as well as the downward movement of the heater coil for zone-melting operation, even when rods of greater diameter are being processed. This results in a considerable saving in time, because it is no longer necessary to turn the semiconductor rod 130 end over end, after one half of the desired total number of zone passes have been completed, and thereafter performing the other halt of the passes with an upward coil-travelling direction th-at is now reversed relative to the rod, the latter method having been necessary for zone-levelling in order to eflect a uniform distribution of impurities over the entire rod length.

The vessel, the two opposite rod holders, means for relative displacement of the rod holders with respect to each other, toward or away from each other, means for relativedisplacement of the entire device comprised of coils 2, 3, 6, and shunt 4 with respect to the holders, by moving said device for example on said holders, and means for rotating one 'or both holders together or separately, have not been illustrated. Should said illustration appear necessary, I reserve right to import into specification and drawing such means as is described in the prior patents, viz. Emeis and Hans, US. Patent 2,904,663, and also that described in the co-assigned application of Schweikert et al., Serial No. 736,387, filed May 19, 1958, now Patent 3,030,189.

The compensation described above can be achieved also by other means such as follow (see FIG. 3). The direction of winding of coil 7 is opposite that of coil 2, so that the respective magnetic fluxes are directed in the opposite sense. This results in opposition of the induced currents in the semiconductor rod, or more properly stated results in zero induction in the rod within the coil 7. A like effect can be obtained by reversing the polarity of the electric connections to coil 7, and keeping the direction of winding of the coils the same.

I claim:

1. An apparatus for floating zone-melting of vertically mounted rods of semiconductor material of more than 15 mm. diameter, comprising two holder members for holding the respective ends of the rod, and an induction heater coil surrounding the rod, the said coil and holder members being relatively movable for relative movement of the coil along the rod, a closed ring of elec trically conducting material surrounding the semiconductor rod, high-frequency lead in means connected to said heater coil and bypassingsaid closed ring, said ring being located beneath the heater coil at such a fixed distance therefrom as to induce in said ring a counter current suilicient for "opposing in the central region encompassed by said ring the flux of said heater coil, said induction heater coil comprising an annular cooling tube having connections for passing a cooling fluid theretube being coiled about and being joined to said ring or heat conduction therewith.

2. An apparatus according to claim 1 characterized in that the ring consists of silver.

3. An apparatus for floating zone-melting of verticaliy mounted rods of semiconductor material, comprising two boider members for holding respective end portions of the rod, and an induction heater coil surrounding the rod, the said coil and holder members being relatively movable for relative movement of the coil along the rod, a ring of electrically conducting material surrounding the semiconductor rod and located beneath the heater coil at a fixed distance therefrom, said induction heater coil comprising an annular first cooling tube having connections for passing a cooling fluid therethrough, said cooling tube having electric connectinos for passing high-frequency electric alternating current therealong, an annular second cooling tube connected for passage of cooling fluid therethrough, the second cooling tube being coiled about and being joined to said ring :for heat conduction therewith, and an electric shunt connected for bypassing the high-frequency electric alternating current around the second cooling tube, and in consequence around the said ring.

4. An apparatus for floating zone-melting of verticaily mounted rods of semiconductor material, comprising two holder members for holding the respective ends of the rod, an induction heater coil surrounding the red, the said coil and holder members being relatively move 'le for relative movement of the coil along the rod, a closed ring of electrically conducting material surrounding the semiconductor rod and located beneath the heater coil at a fixed distance therefrom, said heater coil inducing in said ring electric currents producing a magnetic field in the central region encompassed by the ring of opposite polarity to the magnetic field produced in said central region by the induction heater coil, said induction heater coil comprising an annular cooling tube having connections for passing a cooling fluid therethrough, an annular second cooling tube connected for passage of cooling fluid therethrough, the second cooling tube being coiled about and being joined to said ring for heat conduction therewith, and an electric shunt connected for by-passing the high-frequency electric alternating current around the second cooling tube, and in consequence around the said ring.

5 An apparatus for floating zone-melting o'f vertically mounted rods of semiconductor material, comprising two holder members for holding respective end portions of the rod, and an induction heater coil'surrounding the red, the said coil and holder members being relatively movable for relative movement of the coil along the rod, a ring of electrically conducting material surrounding the semiconductor rod and located beneath the heater coil at a fixed distance therefrom, said induction heater coil comprising an annular first cooling tube having connections for passing a cooling fluid therethrough, said cooling tube having electric connections for passing high-frequency electric alternating current therealong, an annular second cooling tube connected for passage of cooling fluid therethrough, the second cooling tube being coiled about and being joined to said ring for heat conduction therewith, and an electric shunt connected for bypassing thehigh'frequency electric alternating current around the second cooling tube, and, in consequence around the said ring, the induction heater coil being a flat planar, hollow tubular coil con stituting said first cooling tube and having, in the axial direction, a singie layer of spirally wound turns.

6. An apparatus for floating zone-melting of vertically mounted rods of semiconductor material, comprisalong the rod, a ring of electrically con-ducting material surrounding the semiconductor rod and located beneath the heater coil, said induction heater coil comprising an annular first cooling tube having connections for passing a cooling fluid therethrough, said cooling tube having electric connections for passing high-frequency electric alternating current therealong, an annular second cooling tube connected serially to the first cooling tube for passage of said cooling fluid therethrough, the second cooling tube being coiled about and being joined to said ring for heat conduction and an electric shunt connected for bypassing the high-frequency electric alternating current around the second cooling tube, and in consequence around the said ring, the induction heater coil being axially short with respect to the space between said holder members and being a flat planar, hollow tubular coil constituting said first cooling tube and having, in the axial direction, a single layer of spirally wound turns, the second cooling tube constituting, in the axial direction, a plurality of layers of spirally wound turns.

7. An apparatus for floating zone-melting of vertically mounted rods of semiconductor material, comprising two holder members for holding respective end portions of the rod, and an induction heater coil surround ing the rod, the said coil and holder members being relatively movable for relative movement of the coil along the rod, a closed ring of electrically conducting material surrounding the semiconductor rod and located beneath the heater coil at a fixed distance therefrom, said induction heater coil comprising an annular first cooling tube having connections for passing a cooling fluid therethrough, said cooling tube having electric connections for passing high-frequency electric alternating current therealong, an annular second cooling tube connected serially to the first cooling tube for passage of said cooling fluid therethrough, the second cooling tube being coiled about and being joined to said ring for heat conduction therewith, and an electric shunt connected for bypassing the high-frequency electric alternating current around the second cooling tube, and in consequence around the said ring, the induction heater coil being axially short with respect to the space between said holder members and being a flat planar, hollow tubular coil constituting said first cooling tube and having, in the axial direction, a single layer of spirally wound turns, the second cooling tube constituting, in the axial direction, a plurality of layers of spirally Wound turns, the axial length of the latter turns and of the ring being not more than the internal diameter of the ring, the inner diameter of the ring being not more than the inner diameter of said heater coil.

8. An apparatus for floating zone-melting of vertically mounted rods of semiconductor material, comprising two holder members for holding the respective ends of the rod, and an induction heater coil surrounding the red, the said coil and bolder members being relatively movable for relative movement of the coil along the rod, an annular member of electrically conducting material surrounding the semiconductor rod and located beneath the heater coil, at a distance for inducing in said annular member electric currents producing a magnetic field in the interior region encompassed by the member of opposite polarity to the magnetic field produced in said region by the induction heater coil, said induction heater coil forming a first part of cooling tube means having connections for passing a cooling fluid therethrough, said cooling tube means having a second part communicating with said first part and comprised of a plurality of layers of spirally wound tube turns in the axial direction connected with said annular member for heat conduction, said coil constituting in the axial direction a single layer of tube turns, and an electric shunt connected across the second part for bypassing electric currents around said annular member.

9. An apparatus for floating zone-melting of an upstanding rod of semiconductor material, comprising means for supporting the rod, an induction heater coil surrounding the rod, the induction heater and the rod being relatively movable, for relative movement of the heater lengthwise of the rod, an annular member inductively coupled with said heater coil surrounding the rod and located beneath and near the heater coil, so as to control the shape of the lower part of the molten Zone, said induction heater coil forming a first part of cooling tube means having connections for passing cooling fluid therethrough, said cooling tube means having a second part communicating with the first part and connected to said annular member for heat conduction, said cooling tube means being adapted for connection to a high frequency current source, and shunt means connected across the second part for bypassing electric currents around said annular member.

References Cited in the file of this patent UNITED STATES PATENTS 2,678,371 Andrew May 11, 1954 2,870,309 Capita Jan. 20, 1959 2,904,663 Emeis et al Sept. 15, 1959 2,905,798 Freutel Sept. 22, 1959 3,030,189 Schweikert Apr. 17, 1962 

9. AN APPARATUS FOR FLOATING ZONE-MELTING OF AN UPSTANDING ROD OF SEMICONDUCTOR MATERIAL COMPRISING MEANS FOR SUPPORTING THE ROD, AN INDUCTION HEATER COIL SURROUNDING THE ROD, THE INDUCTION HEATER AND THE ROD BEING RELATIVELY MOVABLE, FOR RELATIVE MOVEMENT OF THE HEATER LENGTHWISE OF THE ROD, AN ANNULAR MEMBER INDUCTIVELY COUPLED WITH SAID HEATER COIL SURROUNDING THE ROD AND LOCATED BENEATH AND NEAR THE HEATER COIL, SO AS TO CONTROL THE SHAPE OF THE LOWER PART OF THE MOLTEN ZONE, SAID INDUCTION HEATER COIL FORMING A FIRST PART OF COOLING TUBE MEANS HAVING CONNECTIONS FOR PASSING COOLING FLUID THERETHROUGH, SAID COOLING TUBE MEANS HAVING A SECOND PART COMMUNICATING WITH THE FIRST PART AND CONNECTED TO SAID ANNULAR MEMBER FOR HEAT CONDUCTION, SAID COOLING TUBE MEANS BEING ADAPTED FOR CONNECTION TO A HIGH FREQUENCY CURRENT SOURCE, AND SHUNT MEANS CONNECTED ACROSS THE SECOND PART FOR BYPASSING ELECTRIC CURRENT AROUND SAID ANNULAR MEMBER. 